The present invention relates to an apparatus for generating P waves as well as S waves in a ground formation for the purpose of a geological survey and the like.
As is well known to those skilled in the art, the elastic or seismic waves propagated through the ground consist primarily of two types, namely longitudinal waves (P waves) and shear waves (S waves), wherein the shear wave is known to be a seismic body wave propagated in the deep formation of the earth by a shearing motion of material and is also known as a distortional wave. The terms P wave and S wave are used throughout the application to respectively mean the longitudinal wave and the shear wave of the elastic waves.
In recent years accurate data of the elastic waves have been required for the purpose of seismic or geological surveys in various fields such as civil engineering, architectural engineering, etc., wherein the propagation speed of P waves and S waves is directly measured within a bore hole so that necessary geological characteristics can be obtained. At present, however, a device for detecting only P waves is practically available. This is due to the fact that while a P wave is readily recognized because of its faster propagation and larger amplitude than S waves, the S wave is often influenced and disturbed by the P wave, resulting in a difficulty in the recognition or discrimination of an S wave.
An attempt has been made to effectively radiate only S waves while minimizing the radiation of P waves, as disclosed in U.S. Pat. No. 4,207,961 to Kitsunezaki, issued June 17, 1980, and assigned to the present assignee, in which an exciting method of S waves is provided. The method incorporates inserting a wave source into a bore hole which contains water and actuating an operation device in the wave source to impart an exciting force to the bore hole wall through the water in the direction perpendicular to the axis of the bore hole, thereby generating an S wave which has characteristics of predominant directivity of radiation in the direction perpendicular to the axis of the exciting force. In the method, the water is abruptly ejected at one side of the wave source intersecting the axis of the exciting force while the water of equivalent volume is abruptly simultaneously sucked in at its opposite side. Thus, a positive and negative pressure change is indirectly applied through the medium of the water to the two respective sides of the bore hole wall intersecting the exciting force axis.
The aforementioned method will fulfill the requirements, and while it has met with some degree of success to a large extent, the method and apparatus disclosed in the aforementioned U.S. patent have resulted in some disadvantages. To be more specific, there is a case which will necessitate the surveying of both P waves and S waves simultaneously in order to obtain an accurate data of the ground formation, as is required in such a case that geophysical quantities (i.e., propagation speed) based upon the P waves as well as the S waves is surveyed to obtain different quantities such as Poisson's ratio, by utilizing the surveyed geophysical quantities. If the waves source for only S waves is employed, an additional device or wave source for P waves must be independently installed at the other place of a probe while the wave source for S waves is attached to one place of the probe, or otherwise a separate survey should be conducted by lowering into the bore hole the probe for P waves after the probe for S waves is removed from the bore hole. However, the device disclosed in the aforementioned U.S. patent does not ensure an excitement of P waves and S waves at the same place, and moreover, it is more difficult to apply an exciting force if the position to be surveyed is deep in the ground. Furthermore, a surveying operation of P waves and S waves by different sources for P waves and for S waves will result in unreliability of geophysical quantities which have been obtained based upon the propagation speed of P waves and S waves, and results in difficulty in the evaluation of data errors obtained by the different wave sources which are possibly located at different places in the bore hole.